Thermal generation of mask pattern

ABSTRACT

A mask having a first region and a second region; the first region having a multilayer mirror over a substrate, the multilayer mirror having alternating layers of a first material and a second material, the first material having a high index of refraction, the second material having a low index of refraction; and the second region having a compound of the first material and the second material over the substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of semiconductor integratedcircuit manufacturing, and more specifically, to a method of creating apattern on an EUV mask blank without particles and an EUV mask blankmade with such a method.

2. Discussion of Related Art

During the fabrication of integrated circuits (IC), each semiconductorwafer is subdivided into small identical fields that are adjacent toeach other. Each field includes one or more IC chips. Aradiation-sensitive material, such as photoresist, is coated onto thewafer. Then a scanner with a reduction projection system is used to scanradiation across a mask and onto one field at a time to expose portionsof the photoresist. The mask determines the pattern to be transferred tothe wafer through the process of exposing, developing, and etching.

Deep ultraviolet (DUV) lithography uses a transmissive mask at 248nanometers (nm), 193 nm, or 157 nm to print features with a criticaldimension (CD) of 100-180 nm. Next generation lithography (NGL), such asextreme ultraviolet (EUV) lithography, can print features with smallerCD. For example, an EUV scanner may use 4 imaging mirrors and aNumerical Aperture (NA) of 0.10 to achieve a CD of 50-70 nm with a depthof focus (DOF) of about 1.00 micrometer (um). Alternatively, an EUVscanner may use 6 imaging mirrors and a NA of 0.25 to print a CD of20-30 nm although the DOF will be reduced to about 0.17 um.

EUV lithography uses a reflective mask for exposure since nearly allcondensed materials are highly absorbing in the EUV wavelength range of11-15 nm. An EUV mask may be fabricated from an EUV mask blank after theEUV mask blank has been inspected for defects and repaired. In order toidentify the location of the defects, a pattern, such as a fiducialmark, is etched into the reflective surface of an EUV mask blank.However, etching produces particles that may contaminate tools anddegrade process yield.

Thus, what is needed is a method of creating a pattern on an EUV maskblank without generating particles and an EUV mask blank made with sucha method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a cross-sectional view of an EUV maskhaving a mark according to the present invention.

FIGS. 2(a)-(c) are illustrations of a cross-sectional view of a methodof heating to fabricate a mark on an EUV mask.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In the following description, numerous details, such as specificmaterials, dimensions, and processes, are set forth in order to providea thorough understanding of the present invention. However, one skilledin the art will realize that the invention may be practiced withoutthese particular details. In other instances, well-known semiconductorequipment and processes have not been described in particular detail soas to avoid obscuring the present invention.

A mask is used in lithography to define a desired pattern in photoresiston a wafer. Extreme ultraviolet (EUV) lithography uses a reflective maskfor exposure since nearly all condensed materials are highly absorbingin the EUV wavelength range of 11-15 nm. An EUV mask is fabricated froman EUV mask blank after the EUV mask blank is inspected for defects.Fiducial marks are needed on the EUV mask blank to serve as a referencefor identifying the location of the defects. The present inventionincludes a method of creating a pattern, such as a fiducial mark, on anEUV mask blank without generating particles and an EUV mask blank madewith such a method.

FIG. 1 shows an embodiment of an EUV mask blank 1000 according to thepresent invention. The EUV mask blank 1000 has a first region 1010 and asecond region 1020. The first region 1010 is a multilayer mirror 1200disposed over a substrate 1100. The multilayer mirror 1200 has 20-80pairs of alternating layers of a high index of refraction material 1210,such as Molybdenum, and a low index of refraction material 1220, such asSilicon.

The second region 1020 is a compound, such as Molybdenum Silicide, ofthe high index of refraction material 1210 and the low index ofrefraction material 1220 that is disposed over a substrate 1100. Theupper surface of the second region 1020 is lower than the upper surfaceof the first region 1010. The difference in step height between thefirst region 1010 and the second region 1020 defines a feature 1250 in apattern, such as a fiducial mark.

Various embodiments of a method of creating a pattern, such as afiducial mark, on an EUV mask blank 1000 according to the presentinvention will be described next.

First, a substrate 1100 having a low coefficient of thermal expansion(CTE), a smooth surface, and a low defect level is used as the startingmaterial for an EUV mask of the present invention. An embodiment isshown in FIG. 2(a). The substrate 1100 may be formed out of aglass-ceramic material with the desired properties.

Second, a multilayer (ML) mirror 1200 is formed over the substrate 1100since an EUV mask operates on the principle of a distributed Braggreflector. An embodiment is shown in FIG. 2(b). The ML mirror 1200includes about 20-80 pairs of alternating layers of a high index ofrefraction material 1210 and a low index of refraction material 1220.The thickness uniformity should be better than 0.8% across the substrate1100.

In one embodiment, the ML mirror 1200 includes 40 pairs of alternatinglayers of a high index of refraction material 1210 and a low index ofrefraction material 1220. The high index of refraction material 1210 maybe formed from about 2.8 nm thick Molybdenum while the low index ofrefraction material 1220 may be formed from about 4.1 nm thick Silicon.As needed, a capping layer (not shown), such as about 11.0 nm thickSilicon, may be formed over the ML mirror 1200 to prevent oxidation ofMolybdenum at the upper surface of the ML mirror 1200 in an EUV mask.The ML mirror 1200 can achieve a peak reflectivity of about 60-75% atthe EUV central illumination wavelength of about 13.4 nm.

Ion beam deposition (IBD) or direct current (DC) magnetron sputteringmay be used to form the ML mirror 1200 over the substrate 1100. IBDresults in less perturbation and fewer defects in the upper surface ofthe ML mirror 1200 because the deposition conditions may be optimized tosmooth over a defect on the substrate 1100. DC magnetron sputtering ismore conformal, thus producing better thickness uniformity, but anydefect on the substrate 1100 will tend to propagate up through thealternating layers to the upper surface of the ML mirror 1200.

Third, a beam 1230 is used to locally heat a desired region of the MLmirror 1200. An embodiment is shown in FIG. 2(c). The heating time maybe as short as a few seconds. Heating the desired region of the MLmirror 1200 to a temperature of 300-800 degrees C. will result ininterdiffusion of the thin alternating layers of high index ofrefraction material 1210, such as Molybdenum, and low index ofrefraction material 1220, such as Silicon. The interdiffusion will forma different material, such as a compound of Molybdenum Silicide, thatoccupies a smaller volume than the original ML mirror 1200.

The contraction, or reduction in volume, will create a difference instep height between the heated region and the surrounding unheatedregions of the ML mirror 1200. The difference in step height defines afeature 1250 in a pattern, such as a fiducial mark. If desired the beam1230 may be tilted or rotated to create different slopes or angles inthe sidewalls of the feature 1250 in a pattern. The beam 1230 may becollimated or focused to help obtain the desired profile.

The heating process may be adjusted to vary the extent and rate ofcontraction to avoid any undesirable deformation or other damage thatmay be induced by thermal or mechanical stress. Unlike in an etchingprocess, a heating process does not remove any material so no particleis formed. Any particle is undesirable since it may contaminate a toolor degrade yield.

In one embodiment, the beam 1230 is an electron beam. The range of thesecondary electrons and the scattered electrons may be optimized byadjusting the acceleration voltage. In another embodiment, the beam 1230is an ion beam. Sputtering, ion implantation, and knock-on should beavoided. In still another embodiment, the beam 1230 is an optical beam,such as a laser beam that has a continuous wave or is pulsed. Thetransmission and reflectivity characteristics of the ML mirror 1200should be considered in selecting the appropriate wavelength andbandwidth.

The beam 1230 may be scanned through a field aperture to define afeature 1250 in a pattern that differs in size from the beam 1230.Scanning may also be used to create a feature 1250 in a pattern thatdiffers in shape from the beam 1230. Scanning may be helpful inimproving the uniformity of the heating.

Many embodiments and numerous details have been set forth above in orderto provide a thorough understanding of the present invention. Oneskilled in the art will appreciate that many of the features in oneembodiment are equally applicable to other embodiments. One skilled inthe art will also appreciate the ability to make various equivalentsubstitutions for those specific materials, processes, dimensions,concentrations, etc. described herein. It is to be understood that thedetailed description of the present invention should be taken asillustrative and not limiting, wherein the scope of the presentinvention should be determined by the claims that follow.

Thus, we have described a method of creating a pattern on an EUV maskblank without generating particles and an EUV mask blank made with sucha method.

What is claimed is:
 1. A method comprising: providing a substrate;forming a multilayer mirror over said substrate, said multilayer mirrorhaving a first region and a second region; and heating said secondregion of said multilayer mirror with a beam to create a difference instep height between said first region and said second region, whereinsaid beam is tilted or rotated to create different slopes or angles insidewalls of said second region.
 2. The method of claim 1 wherein saidbeam is an electron beam.
 3. The method of claim 1 wherein said beam isan ion beam.
 4. The method of claim 1 wherein said beam is an opticalbeam.
 5. The method of claim 1 wherein said difference in step heightresults from a contraction or reduction in volume.
 6. The method ofclaim 1 wherein said heating achieves a temperature of 300-800 degreesC.
 7. The method of claim 1 wherein said heating is as short as a fewseconds.
 8. The method of claim 1 wherein said heating of said secondregion of said multilayer mirror results in interdiffusion.
 9. A methodcomprising: providing a substrate; forming a multilayer mirror over saidsubstrate, said multilayer mirror comprising: alternating layers of afirst material and a second material, said first material having a highindex of refraction, said second material having a low index ofrefraction; and forming a third material from a portion of saidmultilayer mirror with a beam, said portion differing in size and shapefrom said beam.
 10. The method of claim 9 wherein said first material isMolybdenum.
 11. The method of claim 9 wherein said second material isSilicon.
 12. The method of claim 9 wherein said third material isMolybdenum Silicide.